The Caliciviridae are a family of positive-strand RNA viruses and consist of four genera designated: (1) Norovirus (with species Norwalk virus);(2) Sapovirus (with species Sapporo virus);(3) Vesivirus (with species, feline calicivirus and vesicular exanthema of swine virus);and (4) Lagovirus (with species rabbit hemorrhagic disease virus and European brown hare syndrome virus). Norwalk virus is the prototype strain for the genus Norovirus and was discovered by LID researchers in 1972. The noroviruses are the major cause of nonbacterial epidemic gastroenteritis that occurs in family, school, institutional, or community-wide outbreaks, affecting all age groups. The noroviruses are also an important cause of sporadic, acute gastroenteritis in infants and young children. The human noroviruses are genetically diverse and cannot be grown in cell culture, which has been a continuing research obstacle.